Absorption Spectrum and Absolute Absorption Cross Sections of

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Absorption Spectrum and Absolute Absorption Cross Sections of CH3O2 Radicals and CH3I Molecules in the Wavelength Range 7473− 7497 cm−1 Eszter P. Faragó,†,‡ Bela Viskolcz,‡ Coralie Schoemaecker,† and Christa Fittschen*,† †

PysicoChimie des Processus de Combustion et de l’Atmosphère PC2A, CNRS - University Lille 1, Cité Scientifique, Bât. C11, 59655 Villeneuve d’Ascq, France ‡ Department of Chemical Informatics, Faculty of Education, University of Szeged, Boldogasszony sgt. 6, Szeged, Hungary 6725 ABSTRACT: The absorption spectrum of CH3O2 radicals and CH3I molecules has been measured in the range 7473−7497 cm−1. CH3O2 radicals have been generated by 248 nm laser photolysis of CH3I in the presence of O2, and the relative absorption has been measured by time-resolved continuous-wave cavity ring-down spectroscopy (cw-CRDS). Calibration of the relative absorption spectrum has been carried out on three distinct wavelengths by carefully measuring CH3O2 decays under different experimental conditions and extracting the initial radical concentration (and with this the absolute absorption cross sections) by using the well-known rate constant for the CH3O2 self-reaction. The following, pressure-independent absorption cross sections were determined: 3.41 × 10−20, 3.40 × 10−20, and 2.11 × 10−20 cm2 at 7748.18, 7489.16, and 7493.33 cm−1. These values are 2−3 times higher than previous determinations (Pushkarsky, M. B.; Zalyubovsky, S. J.; Miller, T. A. J. Chem. Phys. 2000, 112 (24), 10695−10698 and Atkinson, D. B.; Spillman, J. L. J. Phys. Chem. A 2002, 106 (38), 8891−8902). The absorption spectrum of the stable precursor CH3I has also been determined and three characteristic sharp absorption lines with absorption cross sections up to 2 × 10−21 cm2 have been observed in this wavelength range.

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INTRODUCTION Alkylperoxy radicals (RO2) are reactive intermediates in the oxidation processes of hydrocarbons in the troposphere and during low temperature combustion. RO2 radicals are formed by reaction of alkyl radicals with molecular oxygen. In the presence of NOx, peroxy radicals are removed from the atmosphere by the following reaction sequence, considering the simplest case (R = CH3): CH3O2 + NO → CH3O + NO2 (R1) CH3O + O2 → CH 2O + HO2

Considering that CH3O2 is formed as a primary intermediate in the oxidation of CH4, one of the most abundant tropospheric trace gases, as well as during the degradation process of many other larger hydrocarbons, the CH3O2 radical is one of the most abundant peroxy radicals. Therefore, it is of great interest to study the reactivity of CH3O2 radicals and hence a selective and sensitive detection method is necessary. In the past, laboratory experiments have mostly used UVabsorption for the detection of CH3O2 radicals. The absorption spectrum has been measured several times1−3 and a maximal absorption coefficient of 4.6 × 10−18 cm2 at 240 nm has been obtained. However, the absorption spectrum is broad and unstructured and does thus often prevent a selective detection of CH3O2 radicals in the presence of other species. Peroxy radicals have a more structured and characteristic absorption band in the near-infrared due to an A ← X transition. The corresponding absorption spectrum for the CH3O2 radical has been measured several times. A first report on the absorption features of peroxy radicals in the near-infrared was given in 1976 by Hunziker and Wendt using a modulated formation of peroxy radicals by Hg-photosensitized decomposition of appropriate precursors combined with phase sensitive detection of the transient absorption.4 Later the spectrum was published again by Pushkarsky et al.5 using pulsed cavity ring-down spectroscopy (CRDS) coupled to laser photolysis. Pushkarsky

(R2)

HO2 in turn oxidizes another NO to NO2, with NO2 being subsequently photolyzed and leading to the formation of tropospheric ozone. In remote environments, i.e., under low NOx conditions such as the marine boundary layer or remote tropical forests, the reaction with NO becomes too slow and peroxy radicals react predominantly in self-reaction and cross reaction with other peroxy radicals or with HO2: CH3O2 + HO2 → CH3OOH + O2

(R3)

or CH3O2 + RO2 → RO + CH3O + O2

(R4a)

CH3O2 + RO2 → ROH + CH 2O + O2

(R4b)

CH3O2 + RO2 → RC′O + CH3OH + O2

(R4c)

Received: August 30, 2013 Revised: November 1, 2013

where R can be CH3 or any other alkyl group. © XXXX American Chemical Society

A

dx.doi.org/10.1021/jp408686s | J. Phys. Chem. A XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry A

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determination by Atkinson and Spillman.7 They suspect uncertainties in the measurement of the earlier time-resolved traces. They did, however, not remeasure the absorption cross section of CH3O2 by this new method, but it can be suspected that a similar deviation would also be found for this radical. Not only are the absolute absorption cross sections for peroxy radicals in the A ← X transition interesting for a possible selective quantification in laboratory experiments, but also there is the possibility that photolysis of peroxy radicals in the near IR might have some impact on the chemistry of the atmosphere. Indeed, the energy of near IR photons is high enough to induce a possible photolysis of peroxy radicals leading possibly to the formation of OH radicals. Frost et al.9 have carried out model calculations on the impact of such photolysis on the composition of the atmosphere: they modeled three different environments (remote, rural, and urban), and for each environment they made three different cases supposing peak absorption cross sections for four major peroxy radicals (8 × 10−20, 8 × 10−19, and 8 × 10−18 cm2), leading to the formation of OH radicals and aldehydes. It turned out that even with the lower limit for the cross sections, the model predicted a small effect (mostly an increase in aldehyde concentration) on the composition of the atmosphere, particularly in the remote environment where the lifetime of RO2 radicals is long. Therefore, given the uncertainty in the literature on the nearinfrared absorption spectrum of CH3O2 radicals, we measured again the absorption spectrum of the CH3O2 radical using the same technique as Atkinson and Spillman, i.e., laser photolysis coupled to cw-CRDS. CH3O2 radicals have been prepared by 248 nm photolysis of CH3I. The calibration of the spectrum was based on the same method, but now taking into account an additional loss of radicals through diffusion. It turned out that the precursor CH3I also absorbs in this wavelength range and to rule out any impact of a change in CH3I concentration on the absorption spectrum of CH3O2, the absorption spectrum of this stable precursor has also been determined in the wavelength range accessible with our diode.

et al. used a Raman shifted dye laser for generating the near IR radiation and were therefore able to cover a wide wavelength range (7300−7700 cm−1). They have located the 0−0 transition at 7382.8 cm−1 and have obtained at this wavelength an absolute absorption cross section of (2.7 ± 1.4) × 10−20 cm2. They have also reported another absorption structure, assigned to a transition involving the methyl torsion ν12 in the radical, located at 7488 cm−1. This absorption band was later confirmed again by the same group.6 A few years after Pushkarsky et al., the spectrum of the CH3O2 radical was again measured by Atkinson and Spillman,7 demonstrating for the first time the coupling of continuous-wave CRDS (cw-CRDS) to laser photolysis. They used an external-cavity diode (ECD) laser and covered the wavelength range ≈7485−7694 cm−1, corresponding to the ν12 transition. They located its maximum at 7490.24 cm−1 (compared to 7488 cm−1 for Pushkarsky et al.) and reported an absorption cross section of σ = (1.5 ± 0.8) × 10−20 cm2 molecule−1. For obtaining the absorption cross section of this radical, both groups have used the same method: measuring the time-resolved CH3O2 decays and taking advantage of the known rate constant for the self-reaction of CH3O2 radicals to deduce the initial CH3O2 concentration. For determining the kinetic decays, both groups have prepared CH3O2 radicals by the 193 nm photolysis of acetone. A comparison of the absorption cross sections obtained by both groups is not straightforward, because Pushkarsky et al. quantified the cross section of the 0−0 transition, whereas Atkinson and Spillman measured the absorption cross section of the ν12 transition. However, Pushkarsky et al. have measured the absorption spectrum in a wavelength range that covers both peaks, and by assuming that the CH3O2 concentration was constant during the measurement of the entire spectrum, an estimation can be made for the absorption cross section of the ν12 transition in the Pushkarsky et al. spectrum. Doing so, one obtains a rather good agreement of σmax ≈ 1.1 and (1.5 ± 0.8) × 10−20 cm2 for Pushkarsky et al. and Atkinson and Spillman, respectively. Atkinson and Spillman have also measured the absorption cross section of ethylperoxy, C2H5O2, using the same method. In a more recent paper from the same group as Pushkarsky et al. (Melnik et al.8), the absorption spectrum of C2H5O2 radicals has been measured again using a new setup: dual wavelength CRDS coupled to laser photolysis. With this setup they have prepared C2H5O2 radicals by 193 nm photolysis of oxalyl− chlorine in the presence of C2H6 and O2, generating C2H5O2 radicals in the following reaction sequence: C2H6 + Cl → C2H5 + HCl

(R5)

C2H5 + O2 → C2H5O2

(R6)

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EXPERIMENTAL TECHNIQUE CH3O2 radicals have been detected in a time-resolved manner by coupling cw-CRDS to laser photolysis, whereas the spectrum of the stable CH3I molecule, the precursor for the CH3O2 radicals, was measured in the same reaction cell, but without laser photolysis. The detailed description of the timeresolved setup has been published earlier10−12 and the principle of measuring the absorption spectra of stable species has also been described.13 Therefore, only a short summary is given here. The setup is composed of three parts: the photolysis cell, the photolysis excimer laser (Lambda Physic LPX 202i), and the cw-CRD spectrometer. The photolysis cell is a stainless steel tube internally coated with Teflon. The photolysis was carried out by an excimer laser operating at 248 nm. The excimer laser beam (2 cm × 1.5 cm) entered the cell through quartz windows and photolyzed the mixture over the full length of the reactor (70 cm), the energy density used during these experiments was between 10 and 30 mJ cm−2. The cw-CRDS absorption path was installed in a small angle with respect to the photolysis pulse, leading to an overlap between the excimer beam and the cw-CRDS beam of 28.7 cm, calculated from the size of the excimer beam and the crossing angle of the two beams. The near-infrared beam was obtained by a fibered distributed

The absorption of both species, C2H5O2 and the stable byproduct HCl, has been measured simultaneously by the two CRDS absorption paths. Doing so, Melnik et al. do not rely on the self-reaction of C2H5O2 radicals for determining its concentration, but they can measure more precisely the concentration of the stable byproduct HCl. Simultaneously to the quantification of the stable byproduct, Melnik et al.8 used two other methods for deducing the initial radical concentration (a) kinetic decay of C2H5O2 and (b) absorption of the photolysis beam. They find consistent values for the C2H5O2absorption cross sections using all three methods and recommend an absorption cross section of 4.7 × 10−20 cm2 for the 0−0 transition at 7596 cm−1, 50% larger than the earlier B



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feedback (DFB) diode laser (Fitel-Furukawa) and the wavelength range 7473−7497 cm−1 was accessible, i.e., the range of the ν12 transition of the CH3O2 radical. The absorption coefficient α was calculated by the following equation: α = [A] × σ =

RL ⎛ 1 1⎞ ⎜ − ⎟ c ⎝τ τ0 ⎠

(1)

where c is the speed of light, σ is the absorption cross section of the species A, τ and τ0 are the ring-down times in the presence and absence of the absorbing species, and RL is the ratio between the cavity length (L, distance between the two mirrors) and the absorption path length (LA; in the case of photolysis experiments the overlapping of photolysis beam and absorption path is 28.7 cm; in the case of the CH3I spectrum, it is estimated to be 68 cm due to the purge volume in front of cavity mirrors). The ring-down time of the empty cavity τ0 was around 18 μs; i.e., the mirror reflectivity was around 0.99985. All data have been acquired using an acquisition card (National Instruments PC-6259) installed in a personal computer, and all programs were written in the LabView software version 8.2.1. All flows have been controlled by calibrated flow meters. The absorption spectra of two different species, CH3O2 and CH3I, have been measured. Given the fact that the first species is a radical and the second one is a stable molecule, two different strategies have been applied. Time-Resolved Measurement. The absorption spectrum of the CH3O2 radical has been extracted from time-resolved measurements. For this purpose, one time-resolved CH3O2 decay has been measured at each wavelength over a total time span of 1 s before and 1 s after the photolysis pulse. Ring-down events occur randomly and have been accumulated at each wavelength for an undetermined number of photolysis laser shots: the user-set criteria was that at least 20 ring-down events had occurred in the time-window of 10 ms after the photolysis pulse, asking in general for one to three photolysis shots at a repetition rate of 0.2 Hz. This arbitrary criterion ensured that a sufficient number of ring-down events had occurred for a reliable fit of the full decay. A typical example of such a kinetic trace is shown in Figure 1. After accumulation of sufficient ringdown events, the wavelength of the DFB diode was incremented by the LabView program by around 0.01 cm−1 through a change in the current I applied to the DFB diode. The wavenumber in our experiments has been measured by a fibered wavemeter (Burleigh WA-1100) with an accuracy of 0.01 cm−1. To improve the precision of these measurements, the wavenumber ν has been recorded by the LabView program at each 10th increment only and a polynomial fit through the entire data set ν = f(I) has then been used to calculate the wavenumber for each individual measurement. A good accuracy for the wavenumber obtained by this method is verified by comparing the positions of water lines (always present in the reactor through small leaks, but also water in the bath gas helium) in the baseline as well as in the absorption spectra of both CH3O2 and CH3I with the positions of the water lines in high resolution spectra such as found in the literature.14 An excellent agreement is found: for example, the H2O line to be seen at 7484.34 cm−1 in Figure 3 is published at 7484.342 cm−1 by Mikhailenko et al.14 When the entire wavelength range accessible by our DFB diode was covered this way, around 2500 kinetic traces (Figure

Figure 1. Typical time-resolved cw-CRDS signal. Black dots are ringdown events having occurred before the laser pulse and are used to calculate τ0. Green dots are ring-down events having occurred after the photolysis pulse and are used to extrapolate to t = 0. The inset shows a zoom over 50 ms after the photolysis pulse, where the dashed blue line is the biexponential fit for obtaining τt=0.

1) were measured for CH3O2. The portion of the spectrum around the absorption maxima is shown in Figure 2: the baseline τ0 (black line in Figure 2) at a given wavelength was obtained from each kinetic trace from the average over all ringdown events having occurred in the 1 s before the photolysis pulse (black dots in Figure 1). The value τt=0, i.e., the ring-down time just after the photolysis pulse, was obtained by fitting the time-resolved ring-down times τ (green dots in Figure 1) to a biexponential decay (dashed blue line in the inset of Figure 1) up to 200 ms after the photolysis pulse and extrapolation to t = 0 s. Fitting to a biexponential decay has no physical meaning, it just turned out that such fit nicely reproduced the shape of the CH3O2 decay, consisting of a mixture between self-reaction and diffusion at these low initial radical concentrations. From these two values, τ0 and τt=0, the absorption coefficient αt=0 (red line in Figure 2) can then be obtained by applying eq 1. Measuring the spectrum over the entire range of our DFB laser lasted for 2 days, over which the precursor concentration (3.6 × 1014 cm−3) and the laser energy (30 mJ cm−2) were kept constant. CH3I was taken through calibrated flow meters from a

Figure 2. Portion of CH3O2 spectrum around the absorption maxima. Each data point is obtained from Figure 1-type traces: black line obtained from averaging the ring-down events before photolysis laser, green line ring-down time τt=0 obtained by extrapolating a biexponentiel fit of CH3O2 decay to t = 0 (right y-axis applies), and red line absorption spectrum α obtained by applying eq 1 (left y-axis applies). C



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RESULTS AND DISCUSSION CH3O2 Spectrum. In the above section, it has been explained how the absorption coefficient α for CH3O2 has been obtained at each wavelength from one time-resolved CH3O2 kinetic trace. From Figure 2 it can be seen that the ν12 transition of the CH3O2 spectrum comprises a rather broad absorption feature with three distinct maxima, located at 7488.18, 7489.16, and 7490.33 cm−1 : the absorption coefficients at these three wavelengths are summarized in Table 1. The first two maxima exhibit roughly the same

darkened glass bulb, in which a 5% mixture in He had been prepared, the O2 concentration was 7.6 × 1017 cm−3, and the total pressure was 50 Torr. Calibration of the relative spectrum has been done by kinetic experiments. Non-Time-Resolved Spectroscopic Measurements. CH3I is a stable molecule, and hence the photolysis laser was not needed for measuring its spectrum. Therefore, a different technique was used to record the spectrum: although τ and τ0 can be obtained for CH3O2 from the same experiments (before and after laser pulse), for CH3I the baseline τ0 has to be measured separately in the absence of CH3I. This leads to a small complication: the baseline (and also the absorption spectrum) presents a zigzag with a rather long period (in the cm−1 range), as can be seen in Figure 2. This phenomena is due to an etalon effect originating from a parasitic cavity within the cw-CRDS absorption path; i.e., two moderately reflective surfaces (for example, the outer surface of the cavity mirror and one of the lenses) are aligned parallel and lead then to an overlaying sinusoidal signal. The problem is that the period and intensity of this signal is not well reproducible and changes during the day. This of course represents no problem during the measurement of the CH3O2 spectrum, because τ and τ0 are always extracted from the same signal obtained at the same “etalon” position. But this is not true anymore for measuring the spectrum of stable species, because the measurement of τ and τ0 are temporally delayed. To minimize this complication, the baseline measurement has to be done as soon as possible before or after the measurement of the absorption spectrum. For this reason, the spectrum has been measured in small portion of around 4 cm−1, which allowed measuring the absorption spectrum and the baseline within less than 30 min: it has been found that the “etalon” position was stable enough for our purposes over such a time span. For these measurements, 20 ring-down events were acquired before incrementing the wavelength of the DFB diode. Every portion was measured at the same absolute concentrations and pressure as the CH3O2 spectrum. The result is shown in Figure 3 for a small portion:

Table 1. Summary of Absorption Coefficients from Full Spectrum and Absorption Cross Section from Kinetic Measurements at the Wavelengths Corresponding to the Three Distinct Peaks at 50 and 100 Torr ν/cm

−1

7488.18 7489.16 7490.33

−1

αfull spectrum/cm

(4.38 ± 0.3) × 10−7 (4.22 ± 0.3) × 10−7 (2.67 ± 0.3) × 10−7

σCH3O2,50Torr/10−20 cm2

σCH3O2,100Torr/10−20 cm2

3.43 ± 0.21 3.37 ± 0.12 2.13 ± 0.09

3.39 ± 0.04 3.42 ± 0.05 2.12 ± 0.05

absorption coefficient, whereas the third maximum is only half as strong. This relative pattern of the spectrum is in good agreement with the results of Atkinson and Spillman;7 however, the absolute positions of the peaks such as obtained in this work are shifted by around 2 cm−1 toward lower wavenumbers. Compared to the work of the Miller group5,6 (ν12 at 7488 cm−1), the agreement on the position of the absorption maximum is very good, but due to the very low resolution of Figure 1 in Pushkarsky et al., linked to the much larger wavelength range measured in their work, it is not possible to compare details of the shape of this transition. Calibration of CH3O2 Spectrum. A major goal of this work was to determine the absolute absorption cross sections of CH3O2 in this wavelength range. Therefore, we have carried out an absolute calibration of the absorption cross sections at the three wavenumbers corresponding to the above-mentioned distinct absorption maxima. We have proceeded principally in the same way as Pushkarsky et al.5 and Atkinson and Spillman;7 that is, we have deduced the initial CH3O2 concentration from the time-resolved measurement of the concentration-dependent decays. However, we have refined the method by taking into account the irreversible loss of radicals due to diffusion out of the photolyzed volume such as we have already applied to the measurement of the absorption cross sections of HO2 radicals.15 CH3O2 radicals were created by pulsed 248 nm photolysis of CH3I in presence of O2. The main channels consist of the following reactions: CH3I + hν248nm → CH3 + I

Figure 3. Small portion of CH3I spectrum at 50 Torr total pressure: black line, ring-down time with helium only; green line, ring-down time with [CH3I] = 2.0 × 1014 cm−3 (right y-axis applies); red line, α as obtained from eq 1 (left y-axis applies).

CH3 + O2 (+ M) → CH3O2 + (M) 2CH3O2 → 2CH3O + O2 → CH 2O + CH3OH + O2

(R7) (R8) (R9a) (R9b)

The rate constant of reaction R8 is pressure dependent and has been measured in the low pressure limit (1−6 Torr of helium and argon) by Selzer et al.,16 whereas the high pressure limit has recently been determined by Fernandes et al.17 A fit through Selzer’s helium data set using the fall-off expression and k∞,8 from Fernandes et al. leads to a rate constant of

the baseline τ0 as black line, the absorption τ as green line and the absorption spectrum α as red line. The water line at 7484.34 cm−1, clearly visible in the black and green trace, disappears in the red line. D



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k8,50Torr He = 1.4 × 10−13 cm3 s−1. Under our O2 concentrations [(6−8) × 1017 cm−3)], reaction R8 is therefore completed in a few 10 μs. A competition to reaction R8 through the fast CH3 self-reaction (k = 5.99 × 10−11cm3 s−1)18 is non-negligible, especially under our highest initial radical concentrations ([CH3]0 > 1 × 1014 cm−3). However, this reaction has no influence on the subsequent CH3O2 decay, because it takes place on a very short time scale (tens of microseconds) and the product C2H6 does not react with CH3O2. The only result is a lower initial concentration of CH3O2 compared to CH3 (see model in Figure 7). The rate constant for reaction R9 has been measured several times19−23 and is recommended24 as k9 = 3.5 × 10−13 cm3 s−1 at 298 K. The branching ratio between reactions R9a and R9b has been measured twice25,26 and the recommended value24 at 298 K is k9a/k9b = 0.37. The CH3O radicals formed in reaction R9a will induce secondary chemistry: CH3O + O2 → CH 2O + HO2

(R10)

CH3O2 + HO2 → CH3OOH + O2

(R11a)

→ CH 2O + H 2O + O2

m=

and the intercept 1 I= [CH3O2 ]0 × σ

(5)

(R11b)

Figure 4. Typical experiments at 100 Torr. The photolysis energy is around 30 mJ cm−2, ν = 7488.18 cm−1; individual ring-down events are averaged over 5 ms. Key: (a) α = f(t) such as obtained using eq 1 from Figure 1-type graphs; (b) 1/α = f(t) for the same experiments. Full lines are obtained from linear regression of all data between 20 and 200 ms following the photolysis pulse. Resulting initial CH3O2 concentrations (calculated using σ from Table 1) are 10.4, 8.1, and 5.2 × 1013 cm−3.

(2)

The decay can then be approximated as ⎛ kdiff ⎞ 1 1 = +⎜ + 2k 9,obs⎟t [CH3O2 ]t [CH3O2 ]0 ⎝ [CH3O2 ]0 ⎠

(4)

Figure 4a shows three decay curves with different initial CH3I concentrations, and Figure 4b shows the corresponding plots of

with a recommended rate constant24 at 298 K of k11 = 5.2 × 10−12 cm3 s−1. This secondary chemistry leads to an acceleration of the CH3O2 decay, and the apparent, observed rate constant for the decay of CH3O2 radicals through selfreaction has been recommended24 as k9,obs = 4.8 × 10−13 cm3 s−1. A very similar value (4.9 × 10−13 cm3 s−1) has been employed by Pushkarsky et al.5 and Atkinson and Spillman7 to deduce the initial CH3O2 concentration from the time-resolved decays. In the case of laser photolysis, CH3O2 radicals in this system are lost not only through self-reaction (R9) and reaction with HO2 radicals (R11) but also by diffusion out of the observation volume. When the diffusion is approximated to an exponential loss kdiff, CH3O2 concentration−time profiles can be described by the following equation: d[CH3O2 ] = −2k 9,obs[CH3O2 ]2 − kdiff [CH3O2 ] dt

2k 9,obs kdiff + σ [CH3O2 ]0 × σ

these decays to eq 3. It can be seen that the slope increases with decreasing radical concentration due to the increased fraction of radicals lost by diffusion compared to self-reaction. In the example of Figure 4, the slope increases from (3.195 ± 0.07) × 107 over (3.339 ± 0.06) × 107 to (3.646 ± 0.07) × 107 cm s−1. At an infinite high initial radical concentration, the decay should solely be governed by the self-reaction, diffusion being too slow. To separate the self-reaction from the loss through diffusion and extract the absorption cross section from the “real” slope m = 2k9,obs/σ, experiments are carried out for a wide range of initial radical concentrations. From these experiments, the slope m is then plotted as a function of the intercept I such as shown in Figure 5 for all three wavelengths: an extrapolation of the linear regression to I = 0 (i.e., [CH3O2]0 → ∞) leads to a value of m, which is only due to self-reaction. The slope of these plots is linked to diffusion and should therefore depend on the pressure only. To deduce coherent values for all three wavelengths, we have forced the linear regressions at each pressure for all three wavelengths to the same value, namely 14

(3)

where [CH3O2]t is the radical concentration at time t after the photolysis pulse, [CH3O2]0 is the initial radical concentration at time 0 after the photolysis pulse. A plot of eq 3 results in a straight line with the slope being m = (kdiff/[CH3O2]0 + 2k9,obs) and the intercept being I = 1/[CH3O2]0. Therefore, measuring kinetic decays at different initial CH3O2 concentrations allows distinguishing between self-reaction and diffusion: with increasing initial radical concentration, the term k diff/ [CH3O2]0 becomes smaller and the decay approaches the “real” decay solely due to self-reaction. Our goal is to deduce the absorption cross section σ by taking advantage of the known apparent rate constant for the self-reaction, k9,obs. Therefore, we replace the concentration term [CH3O2] in eq 3 by the absorption coefficient α = [CH3O2] × σ, which can be obtained from eq 1. Now, the slope of a plot 1/α = f(t) becomes E



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role under the experimental conditions used by Pushkarsky et al. and are possibly the reason for the lower absorption cross section obtained by this group (see further down for details). Another point to consider is the reactivity of the coproduct of the precursor photolysis, the I atoms. It is known that I atoms recombine rapidly with CH3O2 radicals to form CH3O2I.28 In a recent work, Dillon et al.29 have shown that in a subsequent reaction CH3O2I molecules react very rapidly (1.5 × 10−10 cm3 s−1) with I atoms, leading back to CH3O2 radicals and I2. A model shows that under our conditions this reaction sequence leads to a small decrease in CH3O2 concentration: a steady-state concentration of CH3O2I is established in less than 1 ms and serves afterward as a catalyzer to convert I atoms into I2. This conversion is mostly completed after 2−3 ms and has no impact on the CH3O2 decay kinetic on longer time scales. An increased influence of this secondary chemistry with increasing initial radical concentration was always visible. From signals such as shown in Figure 4 the expected radical concentration can be calculated from the CH3I concentration and the photolysis energy: the agreement is very good at the lowest precursor concentration (5.4 × 1013 cm−3 is expected, 5.2 × 1013 cm−3 is calculated from an extrapolation of α to t = 0 and applying the absorption cross section from Table 1) whereas at the highest precursor concentration 30% of the initial CH3 radicals have not been converted to CH3O2 (15.3 × 1013 cm−3 is expected, 10.4 × 1013 cm−3 is found). However, this does not influence the extraction of the absorption cross section, because the Figure 4b-type data are fitted over a time window of 20−200 ms after the photolysis pulse, a time window where all discussed secondary chemistry (except reactions R10 and R11, taken into account by the use of k9,obs instead of k9) is completed. Pressure Dependence of the CH3O2 Absorption Cross Sections. The CH3O2 absorption spectrum is rather broad, and also the distinct maxima do not consist of sharp lines but are probably due to a congestion of several individual absorption lines. Therefore, a strong pressure dependence of the absorption cross section is not expected. Nevertheless, we have carried out the same set of experiments to obtain absorption cross sections at a total pressure of 100 Torr. In Figure 5, the slope m obtained in these experiments is plotted as a function of the intercept I as filled symbols together with the 50 Torr data (open symbols). It can be seen that, as expected, the slope of the 100 Torr data is flatter than for the 50 Torr experiments, in agreement with a decreased impact of the diffusion with increased pressure. The extrapolation to I = 0 shows, as expected, no pressure dependence of the absorption cross sections. The obtained absorption cross sections for the three wavelengths at 100 Torr are also summarized in Table 1. Also shown in Table 1 are the absorption coefficients α such as obtained from the full spectrum shown in Figure 2. The ratio between α and σ should be constant for all wavelengths and leads to the CH3O2 concentration during the measurement of the full spectrum. Taking the average for all three wavelengths at both pressures, we obtain [CH3O2 ] = 1.1× 1013 cm−3, in very good agreement with the concentration expected from precursor and laser energy measurements (1.26 × 1013 cm−3). Using this concentration allows converting the entire relative absorption spectrum from Figure 2 into an absolute absorption spectrum, shown as red line in Figure 6. Comparison of CH3O2 Absorption Cross Sections with Literature Data. A comparison of the maximal absorption cross

Figure 5. Plot of m as a function of I for all three wavelengths: filled symbols, 100 Torr; open symbols, 50 Torr. For better visibility, error bars (95% confidence interval from Figure 4b-type regressions) are plotted for two data sets only. The dashed line indicates the concentration range used by Pushkarsky et al.,5 and the dotted line indicates the radical concentration used by Atkinson and Spillman7 (valid for green and blue symbols; I value for Atkinson and Spillman concentration for red symbols would be 1.6 × 106 cm).

and 24.8 s−1 for 100 and 50 Torr respectively, obtained as the average of the three slopes from unforced, unweighted regressions. These values are consistent with the slopes obtained in our earlier work on the spectrum of the HO2 radical,15 15 and 41 s−1 at the same pressures: these values are somewhat higher than the values for CH3O2, just as expected due to the smaller size of the HO2 radical compared to CH3O2. This way, absorption cross sections have been extracted for all three wavelengths using a rate constant of k9,obs = 4.8 × 10−13 cm3 s−1. These values are summarized in Table 1. Secondary Chemistry Influencing the CH3O2 Decays. There are other possible side reactions that need to be considered for a reliable extraction of the initial CH3O2 concentration from its kinetic decay. Attention has to be paid to HO2 radicals due to their fast reaction with CH3O2 radicals, as it has been explained already. However, reaction R9a followed by reaction R10 is not the only possible source of HO2 radicals under our conditions, i.e., pulsed generation of relatively high radical concentrations: The fast reaction of CH3 radicals with CH3O2 CH3 + CH3O2 → 2CH3O

(R12)

can take place at short reaction times, i.e., in a time frame when CH3 is still available and considerable amounts of CH3O2 have already been formed. The fraction of CH3 radicals reacting this way rather than through reaction R8 depends on the initial radical and O2 concentration. This reaction with a rate constant27 of k12 = 9.1 × 10−11 cm3s−1 will lead through the subsequent reaction sequence R10 and R11 to an acceleration of the CH3O2 decay (and thereby to an underestimation of the absorption cross section). But this acceleration occurs only during a limited time period until the initially generated additional CH3O radicals from reaction R12 have been used up and the steady-state CH3O concentration obtained from reaction R9a becomes the major “accelerator”. A model shows that under our conditions the additional CH3O from reaction R12 plays some role up to 10−20 ms after the laser pulse, depending on the conditions. But the effect is too small to be visible in Figure 4b, where typical experiments are shown with a linear regression of the data between 20 and 200 ms after the laser pulse. This reaction, however, plays probably a major F



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of high O2 concentrations, HO2 and also CH3COCH2O2 would be formed rapidly and react with CH3O2 radicals (5.2 and 3.8 × 10 −12 cm 3 s −1 for HO 2 and CH3COCH2O2, respectively). Such possible additional loss of CH3O2 radicals due to unidentified side reactions would result in an overestimation of the CH3O2 radical concentration and thus to an underestimation of the absorption cross sections. Unpublished results from our laboratory have shown formation of HO2 radicals following the 248 nm photolysis of CH3I: the yield of this reaction (1−2%) is, however, too small to influence the CH3O2 decays of this work. Also, to generate an underestimation of a factor of 2 in the absorption cross section, the concentrations of unidentified byproducts following the 193 nm photolysis of acetone would need to be inconceivably high and can therefore be excluded as the main reason for the disagreement. (b) Another factor leading to an underestimation of the absorption cross section is the fact that an irreversible loss of radicals due to diffusion out of the observation volume has not been taken into account in the earlier measurements. This effect is negligible in the experiments of Pushkarsky et al.5 for two reasons: (1) they have worked at higher total pressures (200 Torr Ne), which slows down the loss of radicals due to diffusion, and (2) they have used very high initial radicals concentrations i.e., their decays are situated very close to the origin in a Figure 5-type plot (indicated as dashed line in Figure 5) and would lead only to a very minor correction for diffusion at any pressure. This, however, is not true anymore for the experiments such as presented in the work of Atkinson and Spillman:7 the initial concentrations used in their work are somewhat lower (3 × 1013 cm−3) than the concentrations used in this work [(5−10) × 1013 cm−3]. The I value corresponding to

Figure 6. Full absorption spectrum of CH3O2 (red line, right y-axis) and CH3I (blue line, left y-axis).

sections obtained in this work (3.4 × 10−20 cm2) with the two values published in earlier papers (σmax ≈ 1.1 and (1.5 ± 0.8) × 10−20 cm2 for Pushkarsky et al.5 and Atkinson and Spillman7) reveals a strong disagreement. Different reasons are thinkable and three of them will be discussed in the following paragraph: (a) the difference in radical generation, (b) disregard of diffusion as source of CH3O2 loss and (c) very different initial radical concentrations. (a) A major difference is the generation of the peroxy radicals: in the earlier papers (Pushkarsky et al. and Atkinson and Spillman), CH3O2 radicals have been prepared by 193 nm photolysis of the corresponding ketone, whereas in this work CH3I photolysis at 248 nm has been used to prepare CH3O2. The 193 nm photolysis of ketones could bear the danger that unwanted photolytic reactions of the precursor lead to formation of HO2 or other radicals. Takahashi et al.33 have, for example, quantified an H-atom yield of 3.9% following 193 nm photolysis of acetone. Therefore, in the presence

Figure 7. Simulations demonstrating the concentration dependence of the impact of the reaction between CH3 and CH3O2 radicals (reaction R12) on the CH3O2 decay: left panels CH3O2 decay, right panels 1/CH3O2. Upper panel: [CH3]0 = 5 × 1015 cm−3, lower panel [CH3]0 = 1 × 1014 cm−3. G



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Atmospheric Implications. This new, higher absorption cross section can be compared to the cross sections used in the model study of Frost et al.9 on the impact of peroxy radical photolysis on the atmospheric trace gas composition. Supposing that the CH3O2 radical concentration in the work of Pushkarsky et al.5 was constant over the entire wavelength range, one can deduce from our results on the ν12 transition a peak absorption cross section for the 0−0 transition of around 8.6 × 10−20 cm2. This is on the order of the lowest cross sections tested by Frost et al.,9 and for which a small effect on the atmospheric composition has been predicted. The model predicted, for example, an increase of 20−30% for several aldehydes as well as peroxyacetylnitrates and small decreases (up to 20%) in the peroxy radical concentration in a remote location. The work of Frost et al. was carried out in 1999; since then, large progress on the understanding of atmospheric chemistry has been made, and several field campaigns including the measurement of radical concentration have been carried out. Particularly in remote environments with high VOC loads, OH concentrations much higher than predicted30−32 by models have been measured. The underprediction of OH concentration obtained in these campaigns was strongly linked to the NO concentration, i.e., the lifetime of peroxy radicals. Therefore, it would be very interesting to investigate again the role of the near-infrared photolysis of peroxy radicals. CH3I Spectrum. CH3I has been used as a precursor for the generation of CH3O2 radicals. From a decrease in ring-down time after admission of CH3I to the reactor, it was clear that CH3I is absorbing in this wavelength region. A strong absorption of CH3I can induce some error in the measurement of the CH3O2 absorption coefficient: the CH3I concentration decreases following the photolysis pulse, and hence the baseline τ0 that should be used for the calculation of the CH3O2 absorption coefficient would be somewhat higher. This effect, however, is small as only 1−4% of the initial CH3I are photolyzed under our conditions. To verify if this phenomena needs to be taken into account in the analysis of CH3O2 data, the CH3I spectrum has been recorded in the same wavelength range. Figure 3 shows a small portion of the full spectrum: the upper black line represents the spectrum in the absence of CH3I; i.e., only helium at a total pressure of 50 Torr is present in the cell. The change in ring-down by adding CH3I to the gas mixture is represented by the lower green line (right y-axis used for both traces). Both spectra contain water lines (one is visible at 7484.34 cm−1 in Figure 3). The red line represents the absorption coefficient calculated by eq 1 (left y-axis applied). The full CH3I spectrum, now converted to absolute absorption cross sections using a CH3I concentration such as calculated from gas flows and pressure measurements, is shown together with the full CH3O2 spectrum in Figure 7. Three sharp, characteristic peaks were found in this wavelength range at 7473.92, 7481.13, and 7484.42 cm−1 with absorption cross sections of 7.41 × 10−22, 17.3 × 10−22, and 19.0 × 10−22 cm2, respectively. As can be seen in Figure 7, these peaks are located away from the maximum absorption peaks of CH3O2 and the absorption cross sections of CH3I at the wavelengths corresponding to the three CH3O2 peaks do not exceed 2 × 10−22 cm2. With such small absorption cross sections, even the highest photolysis energy, causing a decrease of 4% in the precursor concentration, leads only to a very small change in the baseline (corresponding to a theoretical increase in the ring-

Atkinson and Spillman’s concentration has been indicated for the two bigger peaks (1/(3 × 1013 × 3.3 × 10−20) ≈ 1 × 106) in Figure 5 by a dotted line. It can be seen that not accounting for diffusion under these conditions will lead (in our experimental setup) to an overestimation by a factor of 2 compared to the “real” slope m, and thus an underestimation of the absorption cross section by the same factor. However, the effect of diffusion depends much on the geometry of the experimental setup and the size of the photolyzed volume, it is therefore not to conclude from Figure 5 (based on the geometry of our setup), if diffusion alone can explain the lower absorption cross section observed in Atkinson and Spillman’s experiments. (c) As mentioned, diffusion cannot explain the disagreement with Pushkarsky et al. In this case, secondary chemistry induced by the very high initial radical concentrations might lead to an underestimation of σ if not taken properly into account. With an initial radical concentration of 1016 cm−3 or more, even in the presence of 50 Torr O2, reaction R12 will lead to high initial amounts of CH3O radicals that are converted rapidly to HO2 and that subsequently speed up the CH3O2 decay. Figure 7 demonstrates the impact of reaction R12 on the decay of CH3O2 radicals: simulations have been carried out using a chemical model that contains besides reactions R8−11 the self-reactions of CH3 and HO2 radicals. Two experimental conditions have been simulated corresponding roughly to the conditions of Pushkarsky et al. ([CH3]0 = 5 × 1015 cm−3, [O2] = 2 × 1018 cm−3, k8 = 3 × 10−13 cm3 s−1, upper graphs) and to the conditions used in this work ([CH3]0 = 1 × 1014 cm−3, [O2] = 7 × 1017 cm−3, k8 = 1.4 × 10−13 cm3 s−1, lower graphs). Both conditions have been simulated with (pink trace) and without (green trace) taking into account reaction R12 in the mechanism. From the lower left graph it can be seen that under the conditions used in this work the difference between both decays is very small and the initial CH3O2 concentration is in both cases only slightly lower than the initial CH3 concentration: in the absence of reaction R12 this difference is due to a loss of CH3 radicals by selfreaction and represents less than 10%. The linear regression of both traces in the lower right graph, carried out between 20 and 200 ms just as we have done in this work, leads for both simulations within less than 5% to the expected slope (2 × k9,obs). This is not true anymore for the conditions such as employed by Pushkarsky et al.: one can see from the upper left graph that a large fraction of the CH3 radicals reacts by either self-reaction or through reaction R12: roughly 50% of the CH3 radicals react by self-reaction in the absence of reaction R12, and barely 30% of the initial CH3 radicals are converted to CH3O2 radicals when R12 is added to the mechanism. From the upper right graph it can be seen that under these conditions the addition of reaction R12 to the reaction mechanism has a very strong impact on the CH3O2 decay: the slope of the linear regression, fitted between 0 and 3 ms, is for the green line within 20% of what is expected but increases after addition of reaction R12 to the reaction mechanism more than three times to what would be expected, leading to an underestimation of the absorption cross section of a factor of 3. H



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down time from 18.000 to 18.009 μs). This effect has thus been neglected in the analysis of the CH3O2 data.

(6) Chung, C.-Y.; Cheng, C.-W.; Lee, Y.-P.; Liao, H.-Y.; Sharp, E. N.; Rupper, P.; Miller, T. A. Rovibronic Bands of the Ã Transition of CH3OO and CD3OO detected with Cavity Ringdown Absorption near 1.2−1.4 μm. J. Chem. Phys. 2007, 127, 044311. (7) Atkinson, D. B.; Spillman, J. L. Alkyl Peroxy Radical Kinetics Measured Using Near-infrared CW-Cavity Ring-down Spectroscopy. J. Phys. Chem. A 2002, 106, 8891−8902. (8) Melnik, D.; Chhantyal-Pun, R.; Miller, T. A. Measurements of the Absolute Absorption Cross Sections of the A-X Transition in Organic Peroxy Radicals by Dual-Wavelength Cavity Ring-Down Spectroscopy. J. Phys. Chem. A 2010, 114, 11583−11594. (9) Frost, G. J.; Ellison, G. B.; Vaida, V. Organic Peroxyl Radical Photolysis in the Near-Infrared: Effects on Tropospheric Chemistry. J. Phys. Chem. A 1999, 103, 10169−10178. (10) Thiebaud, J.; Fittschen, C. Near Infrared cw-CRDS Coupled to Laser Photolysis: Spectroscopy and Kinetics of the HO2 Radical. Appl. Phys. B: Lasers Opt. 2006, 85, 383−389. (11) Parker, A.; Jain, C.; Schoemaecker, C.; Szriftgiser, P.; Votava, O.; Fittschen, C. Simultaneous, Time-Resolved Measurements of OH and HO2 Radicals by Coupling of High Repetition Rate LIF and cw-CRDS Techniques to a Laser Photolysis Reactor and its Application to the Photolysis of H2O2. Appl. Phys. B: Lasers Opt. 2011, 103, 725−733. (12) Votava, O.; Masat, M.; Parker, A. E.; Jain, C.; Fittschen, C. Microcontroller Based Resonance Tracking unit for Time Resolved Continuous wave Cavity-Ringdown Spectroscopy Measurements. Rev. Sci. Instrum. 2012, 83, 043110. (13) Jain, C.; Morajkar, P.; Schoemaecker, C.; Viskolcz, B.; Fittschen, C. Measurement of Absolute Absorption Cross Sections for Nitrous Acid (HONO) in the Near-Infrared Region by the Continuous Wave Cavity Ring-Down Spectroscopy (cw-CRDS) Technique Coupled to Laser Photolysis. J. Phys. Chem. A 2011, 115, 10720−10728. (14) Mikhailenko, S.; Kassi, S.; Wang, L.; Campargue, A. The Absorption Spectrum of Water in the 1.25μm Transparency Window (7408 - 7920 cm−1). J. Mol. Spectrosc. 2011, 269, 92−103. (15) Thiebaud, J.; Crunaire, S.; Fittschen, C. Measurement of Line Strengths in the 2v1 Band of the HO2 Radical using Laser Photolysis/ Continous wave Cavity Ring Down Spectroscopy (cw-CRDS). J. Phys. Chem. A 2007, 111, 6959−6966. (16) Selzer, E. A.; Bayes, K. D. Pressure Dependence of the Rate of Reaction of Methyl Radicals with Oxygen. J. Phys. Chem. 1983, 87, 392−394. (17) Fernandes, R. X.; Luther, K.; Troe, J. Falloff Curves for the Reaction CH3 + O2 (+ M) -> CH3O2 (+ M) in the Pressure Range 2 1000 bar and the Temperature Range 300 - 700 K. J. Phys. Chem. A 2006, 110, 4442−4449. (18) Baulch, D. L.; Cobos, C. J.; Cox, R. A.; Frank, P.; Hayman, G.; Just, T.; Kerr, J. A.; Murrells, T.; Pilling, M. J.; Troe, J.; et al. Evaluated Kinetic Data for Combusion Modelling. Supplement I. J. Phys. Chem. Ref. Data 1994, 23, 847−1033. (19) Lightfoot, P. D.; Lesclaux, R.; Veyret, B. Flash Photolysis Study of the Methylperoxy + Methylperoxy Reaction: Rate Constants and Branching Ratios from 248 to 573 K. J. Phys. Chem. 1990, 94, 700− 707. (20) Kurylo, M. J.; Wallington, T. J. The Temperature-dependence of the Rate-constant for the Gas-phase Disproportionation Reaction of CH3O2 radicals. Chem. Phys. Lett. 1987, 138, 543−547. (21) McAdam, K.; Veyret, B.; Lesclaux, R. UV Absorption-Spectra of HO2 and CH3O2 Radicals and the Kinetics of Their Mutual Reactions at 298 K. Chem. Phys. Lett. 1987, 133, 39−44. (22) Cox, R. A.; Tyndall, G. S. Rate Constants for the Reactions of CH3O2 with HO2, NO and NO2 using Molecular Modulation Spectrometry. J. Chem. Soc., Faraday Trans. 1980, 76, 153−163. (23) Jenkin, M. E.; Cox, R. A.; Hayman, G. D.; Whyte, L. J. Kinetic Study of the Reactions CH3O2+ CH3O2 and CH3O2+ HO2 using Molecular Modulation Spectroscopy. J. Chem. Soc., Faraday Trans. 1988, 84, 913−930. (24) Atkinson, R.; Baulch, D. L.; Cox, R. A.; Crowley, J. N.; Hampson, R. F.; Hynes, R. G.; Jenkin, M. E.; Rossi, M. J.; Troe, J. Evaluated Kinetic and Photochemical Data for Atmospheric

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CONCLUSION We have measured the CH3O2 absorption spectrum in the wavelength range 7497.4−7473.8 cm−1 at a total pressure of 50 Torr helium. Radicals have been prepared by 248 nm laser photolysis of CH3I in the presence of O2, and a relative absorption spectrum has been extracted from a time-resolved measurement of CH3O2 decay at each wavelength. Absolute absorption cross sections were determined at two pressures (50 and 100 Torr) for three wavelengths (7488.18, 7489.16, 7490.33 cm−1), corresponding to distinct absorption maxima in the broad absorption band. No pressure dependence was observed, but the absorption cross sections obtained in this work are 2−3 times larger than previously published data: an explanation to this disagreement can possibly be found in neglecting of diffusion by Atkinson and Spillman7 and unaccounted secondary chemistry in the work of Pushkarsky et al.5 The higher absorption cross sections found in this work suggest that the photolysis of peroxy radicals in the nearinfrared region might play some role in the composition of the troposphere, especially in remote environments with low NOx concentrations and hence long RO2 lifetimes. Furthermore, the CH3I spectrum was measured in the same wavelength region.

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AUTHOR INFORMATION

Corresponding Author

*C. Fittschen: e-mail, christa.fi[email protected]; tel, 0033 3 20 33 72 66. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The laboratory PC2A participates in the Institut de Recherche en ENvironnement Industriel (IRENI) which is financed by the Région Nord Pas-de-Calais, the Ministère de l’Enseignement Supérieur et de la Recherche, the CNRS and European Regional Development Fund (ERDF). This project was supported by the French ANR agency under contract No. ANR-11-LabEx-0005-01 CaPPA (Chemical and Physical Properties of the Atmosphere). E.P.F and B.V. acknowlegde for the financial support of TÁ MOP 4.2.2. A-11/1KONV-20120047 and TÁ MOP-4.2.2.C-11/1/KONV-2012-0010 grants.

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REFERENCES

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Absorption Spectrum and Absolute Absorption Cross Sections of

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